Reducing the amount of components having low boiling points in liquefied natural gas

ABSTRACT

Method of reducing the amount of components having low boiling points in liquefied natural gas comprising passing the liquefied natural gas at liquefaction pressure through the hot side of an external heat exchanger to obtain cooled liquefied natural gas, allowing the cooled liquefied natural gas to expand dynamically to an intermediate pressure and statically to a low pressure to obtain expanded fluid, and introducing expanded fluid into the upper part of a fractionation column provided with a contacting section arranged between the upper part and the lower part of the fractionation column; passing a direct side stream at low pressure through the cold side of the external heat exchanger to obtain heated two-phase fluid; introducing the heated two-phase fluid into the lower part of the fractionation column and allowing the vapor to flow upwards through the contacting section; allowing the liquid of the expanded fluid to flow downwards through the contacting section; and withdrawing from the lower part of the fractionation column a liquid product stream having a reduced content of components having low boiling points, and from the upper part of the fractionation column a gas stream which is enriched in components having low boiling points.

The present invention relates to a method of reducing the amount ofcomponents having low boiling points in liquefied natural gas. Thecomponents having low boiling points are generally nitrogen, helium andhydrogen, these components are also called `light components`. In such amethod the liquefied natural gas is liquefied at liquefaction pressure,and subsequently the pressure of the liquefied natural gas is reducedand separated to obtain liquefied natural gas having a reduced contentof components having a low boiling point at a low pressure, whichliquefied natural gas can be further treated or stored. Thus this methodserves two ends, first reducing the pressure of the liquefied naturalgas to the low pressure, and second separating a gas stream includingcomponents having low boiling points from the liquefied natural gas,thus ensuring that the remaining liquefied natural gas has asufficiently low content of components having low boiling points. Ingeneral the contents of low boiling point components, in particularnitrogen, is reduced from between 2 to over 15 mol % to less than 1 mol%. Such a method is sometimes called an end flash method.

The liquefaction pressure of natural gas is generally in the range offrom 3.0 to 6.0 MPa. The low pressure is below the liquefactionpressure, for example the low pressure is less than 0.3 MPa and suitablythe low pressure is about atmospheric pressure, between 0.10 and 0.15MPa.

International patent application publication No. WO 93/08 436 relates toa method of reducing the amount of components having low boiling pointsin liquefied natural gas, which method comprises the steps of:

(a) passing the liquefied natural gas at liquefaction pressure or at anintermediate pressure through the hot side of an external heat exchangerto obtain cooled liquefied natural gas, allowing the cooled liquefiednatural gas to expand to a low pressure to obtain expanded fluid, andintroducing the expanded fluid into the upper part of a fractionationcolumn provided with a contacting section arranged between the upperpart and the lower part of the fractionation column;

(b) passing a liquefied natural gas fraction withdrawn from thefractionation column through the cold side of the external heatexchanger to obtain heated two-phase fluid;

(c) introducing the heated two-phase fluid into the lower part of thefractionation column and allowing the vapour to flow upwards through thecontacting section;

(d) allowing the liquid of the expanded fluid introduced in the upperpart of the fractionation column to flow downwards through thecontacting section; and

(e) withdrawing from the lower part of the fractionation column a liquidproduct stream having a reduced content of components having low boilingpoints, and withdrawing from the upper part of the fractionation columna gas stream which is enriched in components having low boiling points,wherein the expansion from liquefaction pressure to intermediatepressure is done dynamically and wherein the expansion from theintermediate pressure to low pressure is done statically.

The intermediate pressure is in between the liquefaction pressure andthe low pressure, and it is so selected that evaporation during thedynamic expansion is substantially avoided.

In the known method, a fraction is withdrawn from the fractionationcolumn which is heated in the external heat exchanger to provided vapourfor stripping. The fraction is a normal side stream which is removedfrom the fractionation column at a level within the contacting section,which contacting section is arranged below the level at which theexpanded fluid is introduced in the upper part of a fractionationcolumn. For example if the contacting section comprises contactingtrays, the fraction is removed from a level between adjacent contactingtrays. Consequently the fraction has been in intimate contact withvapour rising through the fractionation column before it is removed fromthe fractionation column. A result of this intimate contact is thatmatter and heat are exchanged between the liquid and the vapour. Thusnot only the composition of the liquid is changed but also the liquid isheated.

In the specification the words `gas` and `vapour` will be usedindifferently.

Applicant seeks to improve the above method, and to provide a methodwherein the coldest fluid available is passed through the cold side ofthe external heat exchanger.

To this end the method of reducing the amount of components having lowboiling points in liquefied natural gas according to the presentinvention comprises the steps of:

(a) passing the liquefied natural gas at liquefaction pressure or at anintermediate pressure through the hot side of an external heat exchangerto obtain cooled liquefied natural gas, allowing the cooled liquefiednatural gas to expand to a low pressure to obtain expanded fluid, andintroducing expanded fluid into the upper part of a fractionation columnprovided with a contacting section arranged between the upper part andthe lower part of the fractionation column;

(b) passing a direct side stream at low pressure through the cold sideof the external heat exchanger to obtain heated two-phase fluid, whichdirect side stream is a liquid portion of the liquefied natural gasseparated therefrom at a point which is upstream of the contactingsection in the fractionation column, and suitably separated therefrom ata point which is downstream of the external heat exchanger and upstreamof the contacting section in the fractionation column;

(c) introducing the heated two-phase fluid into the lower part of thefractionation column and allowing the vapour to flow upwards through thecontacting section;

(d) allowing the liquid of the expanded fluid introduced in the upperpart of the fractionation column to flow downwards through thecontacting section; and

(e) withdrawing from the lower part of the fractionation column a liquidproduct stream having a reduced content of components having low boilingpoints, and withdrawing from the upper part of the fractionation columna gas stream which is enriched in components having low boiling points,wherein the expansion from liquefaction pressure to intermediatepressure is done dynamically and wherein the expansion from intermediatepressure to low pressure is done statically.

An advantage of the present invention is that the liquid load in thecontacting section of the fractionation column is reduced, consequentlythe stripping factor is increased and thus the stripping efficiency.

The invention will now be described in more detail with reference to theaccompanying drawings, wherein

FIG. 1 shows a first embodiment of the present invention;

FIG. 2 shows a second embodiment of the present invention;

FIG. 3 shows a third embodiment of the present invention; and

FIG. 4 shows a cross-section of FIG. 3 along the line IV--IV drawn to alarger scale.

Reference is made to FIG. 1. The liquefied natural gas is supplied atliquefaction pressure through conduit 1 to the hot side 2 of externalheat exchanger 3. In the external heat exchanger 3 the liquefied naturalgas is cooled by indirect heat exchange to obtain cooled liquefiednatural gas. The cooled liquefied natural gas is supplied throughconduit 6 to expansion unit 8, which expansion unit 8 comprises a devicefor dynamically expanding liquid in the form of a turbo expander 9 toexpand the cooled liquefied natural gas dynamically from liquefactionpressure to an intermediate pressure and a throttling valve 10 to expandthe cooled liquefied natural gas statically from the intermediatepressure to a low pressure to obtain expanded fluid. The turbo expander9 and the throttling valve 10 are connected by means of connectingconduit 13. The expanded fluid is subsequently supplied through conduit15 to a fractionation column 20 operating at the low pressure.

The expanded fluid is introduced via inlet device 21 into the upper part22 of the fractionation column 20. The fractionation column 20 isprovided with a contacting section 25 arranged between the upper part 22and the lower part 28 of the fractionation column 20. The contactingsection 25 may be formed by a number of axially spaced apart contactingtrays or by packing material to provide intimate contact between gas andliquid, the number of contacting trays or the height of the packingmaterial is so selected that it provided fractionation corresponding tothe fractionation provided by at least on theoretical equilibrium stage,and suitably by between 3 to 10 stages.

In the external heat exchanger 3 the liquefied natural gas is cooled byindirect heat exchange with a direct side stream at low pressure passingthrough the cold side 30 of the external heat exchanger 3 to obtainheated two-phase fluid.

The direct side stream is obtained by taking a portion of the cooledliquefied natural gas at intermediate pressure and allowing it to expandstatically to the low pressure. The portion is removed from the cooledliquefied natural gas at junction 31 and supplied through conduit 32provided with throttling valve 34 to the cold side 30 of the heatexchanger 3.

The heated two-phase fluid is passed at the low pressure through conduit36 to the fractionation column 20, and it is introduced through inletdevice 40 into the lower part 28 of the fractionation column 20. Thevapour from the heated two-phase fluid is allowed to flow upwardsthrough the contacting section 25.

The liquid of the expanded fluid to flow downwards through thecontacting section 25, counter-currently to the vapour.

A liquid product stream containing a reduced amount of components havinglow boiling points is withdrawn from the lower part of the fractionationcolumn 20 through conduit 45, and a gas stream which is enriched incomponents having low boiling points is withdrawn from the upper part ofthe fractionation column 20 through conduit 47.

Because the direct side stream is removed from the cooled liquefiednatural gas at junction 13 it has not been subjected to a fractionation,and therefore it has not been heated. Moreover, because the amount ofliquid flowing downwards through the fractionation column is the amountof liquid in the liquefied natural gas minus the amount of the directside stream, the liquid load in the fractionation column is reduced andconsequently the stripping efficiency is improved.

As shown in FIG. 1 the turbo expanded 9 is arranged downstream of theexternal heat exchanger 3, so that the liquefied natural gas passes atliquefaction pressure through the hot side 2 of the external heatexchanger 3. In an alternative embodiment (not shown) the turbo expanderis arranged upstream of the direct heat exchanger so that the liquefiednatural gas passes at intermediate pressure through the hot side 2 ofthe external heat exchanger 3.

Reference is now made to FIG. 2 showing an alternative embodiment of thepresent invention. The parts which correspond to parts shown in FIG. 1have got the same reference numerals.

The embodiment of FIG. 2 differs only from the one shown in FIG. 1 inthat the direct side stream is obtained in a different way, and theremainder stays the same so that the normal operation will not bediscussed in detail. In the embodiment of FIG. 2, the direct side streamis obtained as follows. A portion of the cooled liquefied natural gas atintermediate pressure is removed from the cooled liquefied natural gasat junction 31 and supplied through conduit 32 provided with throttlingvalve 34 to a separator 50. In the separator 50 vapour is removed fromthe portion and the liquid is passed through conduit 51 to the cold side30 of the heat exchanger 3.

Suitably the vapour is passed through conduit 52 and it is added to theexpanded fluid at junction 53 before it enters into the fractionationcolumn 20.

An improvement of the embodiment of FIG. 2 is now described withreference to FIGS. 3 and 4. The parts which correspond to parts shown inFIG. 1 have got the same reference numerals, and only the operation ofthe different features will be described.

In this improved embodiment, the direct side stream is obtained bywithdrawing a side stream from the upper part 22 of the fractionationcolumn 20. To this end a partial draw-off tray 60 is arranged in theupper part 22 of the fractionation column 20 below the level at whichexpanded fluid is introduced and above the contacting section 25. Thepartial draw-off tray comprises a central trough 62 (see FIG. 4) and aplurality of side troughs 62 opening into the central trough 61. Thefractionation column 20 is provided with an outlet (not shown) forwithdrawing liquid collected by the partial draw-off tray 60.

During normal operation the expanded fluid is introduced into thefractionation column 20 through inlet device 21 and part of the liquiddownflow is collected by the partial draw-off tray 60 and passed as thedirect side stream to the external heat exchanger through conduit 65. Apartial draw-off tray as referred to with reference numeral 60 is a traywhich does not provide intimate gas/liquid contact. Thus the liquidwithdrawn from the tray has the same composition as the liquid enteringthe tray, and consequently vapour and liquid leaving the tray are not inequilibrium with each other. Therefore such a partial draw-off tray isnot a theoretical equilibrium stage.

The amount of direct side stream is between 10 to 60 mol % based on theamount of liquefied natural gas.

An advantage of the method of the present invention over the knownmethod is that the direct side stream, a liquid portion of the liquefiednatural gas separated therefrom at a point which is downstream of theexternal heat exchanger and upstream of the contacting section in thefractionation column, has not been subjected to fractionation so that itis the coldest stream available.

A further advantage of the present invention is that the liquid load inthe contacting section of the fractionation column is reduced,consequently the stripping factor is increased and thus the strippingefficiency.

We claim:
 1. Method of reducing the amount of components having lowboiling points in liquefied natural gas, which method comprises thesteps of:(a) passing the liquefied natural gas at liquefaction pressureor at an intermediate pressure through the hot side of an external heatexchanger to obtained cooled liquefied natural gas, allowing the cooledliquefied natural gas to expand to a low pressure to obtain expandedfluid, and introducing expanded fluid into the upper part of afractionation column provided with a contacting section arranged betweenthe upper part and the lower part of the fractionation column; (b)passing a direct side stream at low pressure through the cold side ofthe external heat exchanger to obtain heated two-phase fluid, whichdirect side stream is a liquid portion of the liquefied natural gasseparated therefrom at a point which is upstream of the contactingsection in the fractionation column, and suitably separated therefrom ata point which is downstream of the external heat exchanger and upstreamof the contacting section in the fractionation column; (c) introducingthe heated-two phase fluid into the lower part of the fractional columnand allowing the vapour to flow upwards through the contacting section;(d) allowing the liquid of the expanded fluid introduced in the upperpart of the fractionation column to flow downwards through thecontacting section; and (e) withdrawing from the lower part of thefractionation column a liquid product stream having a reduced content ofcomponents having low boiling points, and withdrawing from the upperpart of the fractionation column a gas stream which is enriched incomponents having low boiling points, wherein the expansion fromliquefaction pressure to intermediate pressure is done dynamically andwherein the expansion from intermediate pressure to low pressure is donestatically.
 2. Method according to claim 1, wherein the direct sidestream is obtained by taking a portion of the cooled liquefied naturalgas at intermediate pressure and allowing it to expand statically to thelow pressure.
 3. Method according to claim 1, wherein the direct sidestream is the liquid obtained by taking a portion of the cooledliquefied natural gas at intermediate pressure, allowing it to expandstatically to the low pressure to obtain a two-phase fluid, and removingthe vapour from the two-phase fluid.
 4. Method according to claim 3,wherein the vapour is added to the expanded fluid before it is enteredinto the fractionation column.
 5. Method according to claim 1, whereinthe direct side stream is obtained by withdrawing a side stream from theupper part of the fractionation column.